VERTEBRATA, 93 
PuystoLtogy. One of the most marked characteristics of the vertebrate 
animals is that indicated by their common name; they have an internal 
bony skeleton, to which the muscles are affixed, and of which the principal 
part is the backbone, terminated at the anterior extremity by the skull. 
They have in general four limbs, corresponding to the arms and legs of 
man, but very variously modified as organs of mere locomotion, and as 
organs also of prehension ; and assuming not only the characters of arms 
and legs, terminated by hands or feet, but also of fins, wings, &c, Some, 
vertebrate animals have only one pair of limbs instead of two, a few have © 
no limbs at all. No vertebrate animal has any other number of limbs 
than two or four. The limbs are always formed of bones jointed to the 
backbone and one to another, surrounded with muscles, and covered with 
skin. All vertebrate animals have a distinct head, a skull containing the 
brain, and a spinal cord extending through the centre of the backbone. 
They have a system of circulation dependent on the action of a muscular 
heart with two or more cavities. Those which live in water breathe 
by gills, those on dry land by lungs. The digestive system, although 
exhibiting a variety of modifications as great as the external form, presents 
always the same general characters. The mouth, for example, is always 
situated in the front of the head, and has two bony jaws articulated to 
the other bones of the head. A tongue is also always present in the 
mouth, an organ to which nothing truly corresponding is to be found 
in any invertebrate tribe. 
Fishes.—The class of Fishes consists of animals that all live in water, 
and at all stages of their existence breathe by gills. In number—both of 
individuals and of different kinds—fishes are supposed to exceed all 
other vertebrate animals together. Not only the sea, but lakes and 
rivers abound in them, even the waters of hot springs and the pools of 
caverns have their peculiar species. Their form is adapted to easy 
progression through water. The four limbs which belong to the 
vertebrate type are generally all present, assuming the form of fins 
—the first pair, situated on the breast, being the pectoral! fins (p, fig. 70), 
and the second the ventral * (v, fig. 70), their normal position being on the 
belly. ‘The latter are liable to great changes of position, and are often 
found close under the pectoral fins, and sometimes before them, quite on 
the throat of the fish, in which position they are called jugular fins. 
Fishes, however, have also other fins, not so closely connected with the 
proper skeleton, and placed singly on the middle line, both above and 
below. One of these, the caudal * or tail fin (¢, fig. 70), is the principal 
organ of locomotion, the whole muscular power of great part of the 
1 From Latin pectus, the breast. 3 From Latin jugulum, the throat. 
2 From Latin venter, the belly. 4 From Latin cauda, a tail. 
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